The temperature indicated by a glass bulb thermometer whose surface is kept wet by a thin film of liquid, usually water, and which is exposed to a current of (p. 411) air. It is therefore the dynamic equilibrium temperature attained by a liquid surface subject to the action of a rapid stream of gas. The use of both wet and dry bulb temperature is used to determine the humidity of a gas such as water vapour in air for a particular temperature and pressure. The difference between the wet and dry bulb temperatures is known as the wet bulb depression.